Wind turbines typically include a rotor with large blades driven by the wind. The blades convert the kinetic energy of the wind into rotational mechanical energy. The mechanical energy usually drives one or more generators to produce electrical power. Thus, wind turbines include a power transmission system to process and convert the rotational mechanical energy into electrical energy. The power transmission system is sometimes referred to as the “power train” of the wind turbine. The portion of a power transmission system from the wind turbine rotor to the generator is referred to as the drive train.
Oftentimes it is necessary to increase the rotational speed of the wind turbine rotor to the speed required by the generator(s). This is accomplished by a gearbox between the wind turbine rotor and generator. Thus, the gearbox forms part of the power transmission system and converts a low-speed, high-torque input from the wind turbine rotor into a lower-torque, higher-speed output for the generator.
Transmitting torque is not the only function of a wind turbine power transmission system. The secondary function is to transfer other rotor loads to a nacelle structure and tower supporting the system. Indeed, the wind turbine rotor experiences a variety of loads due to variable wind conditions, dynamic interactions, control aspects, gravity, and other factors. The path of these loads through the power transmission system depends on the particular arrangement. Although components are designed with the corresponding load path in mind, the unpredictability, variety, and magnitude of the loads makes this very challenging. Moreover, even properly designed components may not accurately take into account machine tolerances, load deformations, thermal expansions/variations, and other conditions. These conditions may result in undesirable, “parasitic” forces that have the potential to damage elements in the power transmission system, particularly the gearbox components and the main bearing(s). As a result, gearbox and bearing reliability is one of the biggest concerns in the wind power industry.
Some manufacturers address gearbox concerns by designing power transmission systems without a gear stage. The wind turbine rotor directly drives a low-speed generator in such systems. Although the number of components subject to rotor loads may be reduced, these direct-drive wind turbines have the same challenges with respect to parasitic loads in main bearing(s) as well as in the generator components. Direct drive wind turbines also present other concerns. In particular, the low-speed generators are larger than their high and medium-speed counterparts in geared solutions to produce equivalent amounts of power. The larger size presents transportation, assembly, and maintenance challenges in addition to cost concerns, as most direct-drive machines are permanent magnet generators incorporating rare earth materials of limited availability. Moreover, there is also a critical requirement of low tolerances in the generator and controlled management of parasitic forces.
Thus, power transmission systems with a gear stage are still considered to be of interest, and solutions to address the reliability concerns are highly desirable.